Week 8

Question 1

What is a bus?

Answer 1

A bus is a common set of wires that multiple devices share

Question 2

What is a port?

Answer 2

A port is a connection point a device uses to connect to a computer system.

Question 3

What is a controller?

Answer 3

A controller is a collection of electronics that can operate a port, a bus, or a device. For example, a serial-port controller is a simple device controller

Question 4

What is polling?

Answer 4

Polling is where the CPU keeps on checking if the device needs attention. Polling becomes inefficient when it is attempted repeatedly yet rarely finds a device ready for service, while other useful CPU processing remains undone

Question 5

What does an interrupt occur?

Answer 5

An interrupt occurs when the I/O device notifies the CPU that it requires its attention. The CPU will then hold its current task and starts executing the corresponding interrupt.

Question 6

What is the interrupt request line?

Answer 6

The interrupt request line is a wire in the CPU hardware that the CPU senses after executing every instruction. When the interrupt-request line is triggered by an I/O device the CPU performs a state save and jumps to the interrupt-handler routine. The device controller raises an interrupt by asserting a signal on the interrupt request line, the CPU catches the interrupt and dispatches it to the interrupt handler, and the handler clears the interrupt by servicing the device. Interrupt-driven I/O is now much more common than polling

Question 7

What is DMA?

Answer 7

Direct memory access is the process of transferring data without the involvement of the CPU itself. This enables the device to perform tasks without interrupting the CPU. A DMA controller manages the data transfers between memory and the various devices. The CPU tells the DMA controller the source and destination of the transfer and size of data then goes on with other work.

Question 8

What is application I/O interface?

Answer 8

I/O system encapsulates device behaviours in generic classes
 Device-driver layer hides differences among I/O controllers
 New devices taking already-implemented protocols need no extra work
 Each OS has its own I/O subsystem structures and device driver frameworks.

Question 9

What are block and character devices?

Answer 9

1. Block devices include things like disk drives and their commands include read, write, and seek
2. Character devices include keyboards, mice, and serial ports their commands include get(), put().

Question 10

What are network devices?

Answer 10

Network devices vary enough from block and character devices enough to have their own interface.

Question 11

What is buffering?

Answer 11

Buffering is where data is stored in memory while transferring between devices to cope with:
- Device speed mismatch
- Device transfer mismatch
- To maintain “copy semantics”

Question 12

What is caching?

Answer 12

Caching is a region of fast memory that holds a copy of the data and is key to performance.

Question 13

What is spooling?

Answer 13

Spooling is holding output for a device if the device can serve only one request at a time.

Question 14

How can devices and I/O transfers fail?

Answer 14

Transient reasons - network becomes overloaded
Permanent reasons - disk controller becomes defective

Question 15

What are the three types of data files?

Answer 15

1. Numeric
2. Character
3. Binary

Question 16

What are the different file attributes?

Answer 16

Name – only information kept in human-readable form
Identifier – unique tag (number) identifies file within file system
Type – needed for systems that support different types
Location – pointer to file location on device
Size – current file size
Protection – controls who can do reading, writing, executing
Time, date, and user identification – data for protection, security, and usage monitoring

Question 17

What are the two access methods?

Answer 17

1. Sequential access - Information in the file is processed in order, one record after the other. This mode of access is by far the most common
2. Direct Access - The direct access method is based on a disk model of a file since disks allow random access to any file block, For direct access, the file is viewed as numbered sequence of blocks or records.

Question 18

How are directories organised?

Answer 18

Directories are organised logically to obtain:
- Efficiency - locating a file quickly
- Naming - convenient to users
- Grouping - logical grouping of files by properties

Question 19

What are the different types of directories?

Answer 19

1. Single-level directory - Used by all users, consequently has naming problem
2. Two-level directory - There are separate directories for each user, files can have the same name but must be under different user directory. Also has efficient searching
3. Tree-structured directory -